Introduction

Tropical cyclones (TCs) are a particular class of rotating low-pressure systems that develop over tropical and subtropical waters. The systems have a warm-core, a well-organized convection, and cyclonic surface wind circulation (Anthes 1982; Landsea 2000).

Empirical observations (La Seur and Hawkins 1963; Hawkins and Rubsam 1968; Holland 1980; Willoughby 1990, 1991; Vickery et al. 2000; among others) show that in the altitude range from 2-3 km to about 10 km, the tangential winds are in approximate gradient balance and the radial inflow is negligible. Based on earlier work by Schloemer (1954) and Myers (1957), Holland (1980) used a symmetric pressure distribution to derive the tangential gradient wind Vgr, as a function of distance R from the TC center. His result, which we refer to here as Holland's wind profile, is

where Vmax, Rmax, and B are TC-specific constants. The tangential velocity Vgr increases with R to a maximum Vmax at R = Rmax (usually referred to as the radius of maximum winds). For R >> Rmax, Vgr has an approximately power-law decay with distance, with exponent —B/2. According to Willoughby and Rahn (2004), B varies in the range [1, 2] with typical values around 1.4.

Inside the TC boundary layer (BL) (within approximately 1-2 km from the surface), frictional stresses are important and result in an inward net force that drives low-level convergence. Consequently, the horizontal and vertical wind fields are strongly coupled and Eq. (1) does not apply. Horizontal convergence drives the vertical winds, which are maximum at the top of the boundary layer near the radius of maximum winds Rmax (e.g. Kepert 2001 and Kepert and Wang 2001).

Since the convergence of moisture inside the BL is of major importance for the maintenance, evolution and destructive potential of TCs (Emanuel 1986, 1989; Renno and Ingersoll 1996), a number of studies (Myers and Malkin 1961; Chow 1971; Shapiro 1983; Kepert 2001) have focused on developing theoretical models for the boundary layer of moving TCs. These models derive the radial and tangential winds inside the boundary layer from an assumed radial profile of the tangential wind velocity under gradient balance, for example the profile in Eq. (1), and from suitable surface boundary conditions.

Review of Boundary Layer Models reviews these BL models and their limitations. Proposed Model describes our proposed model by giving the governing equations (an extension of the equations of Smith 1968) and discussing their numerical solution. Model Comparison, we compare model results with earlier models and with simulations using the Fifth-Generation Pennsylvania State University/NCAR Mesoscale Model (MM5). Sensitivity Analysis shows how the calculated winds depend on various storm parameters. Conclusions are stated in Conclusions.

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